liii 



cient importance for the separation of Melania dej^y- 

 gis, or of Gijrotonia* from Ancylotns. 



Pachycliili. There is in this group also a 

 marked distinctness of form. As we have excluded 

 this genus from the flxmily Stre2)omatidce on consid- 

 erations entirely conchological, it is very interesting 

 to find in the dentition differences quite as marked 

 as those existing in the shell. To show the very 

 peculiar form of the Rhachidian tooth, wo copy from 

 Troschel the following for comparison : — 



Fig. 23. PachychUus kevisswius. 



Fig. 24. " Scliiedeanus. 



It is curious, however, and shows how little 

 dependence can be placed on any one character in 

 the grouping of MoUusca, to find Pirena and Melan- 

 opsis placed by this author together with Pachy- 

 chUus, on account of their almost identical dentition, 

 when they differ so much in conchological char- 

 acters and in geographical distribution. 



Dr. WiUiam Stimpson, nearly two years sinc^, 

 published a paper in the "American Journal of 

 ticience and Arts, ''On the Structural Characters 

 of the so-called Melanians of North America," con- 

 taining the results of observations of the animals of 

 several of our species, including an /o, Ancidosa, 

 and Goniohasis. The individuals of these three very 

 distinct genera were not found to differ one from 

 another in any structural character, although readily 

 distinguished from Oriental species. We will state 

 the differences in their relative importance, as they 

 appear to us. 1st. By being oviparous, while the 

 latter are ovo-viviparous. 2d. By the mantle- 

 margin being plain in the American, and fringed in 

 the exotic family. 3d. By difference in dentition. 

 To these may be added a sufficient conchological 

 difference to justify the separation into two families, 

 even if the soft parts were undistinguishable. 



* Uc curiously regrets that the uearly-allied geuus ScJiizostoma, Lea, 

 is uukuowQ to him ! 



